Calcium ATPase, E2-Pi state
The P-type ATPases, also known as E1-E2 ATPases, are a large group of evolutionarily related ion and lipid pumps that are found in bacteria, archaea, and eukaryotes. P-type ATPases fall under the P-type ATPase (P-ATPase) Superfamily (TC# 3.A.3) which, as of early 2016, includes 20 different protein families. P-type ATPases are α-helical bundle primary transporters named based upon their ability to catalyze auto- (or self-) phosphorylation of a key conserved aspartate residue within the pump and their energy source, adenosine triphosphate (ATP). In addition, they all appear to interconvert between at least two different conformations, denoted by E1 and E2.
Most members of this transporter superfamily catalyze cation uptake and/or efflux, however one subfamily (TC# 3.A.3.8) is involved in flipping phospholipids to maintain the asymmetric nature of the biomembrane.
Prominent examples of P-type ATPases are the sodium-potassium pump (Na+/K+-ATPase), the plasma membrane proton pump (H+-ATPase), the proton-potassium pump (H+/K+-ATPase), and the calcium pump (Ca2+-ATPase).
The first P-type ATPase discovered was the Na+/K+-ATPase, which Nobel laureate Jens Christian Skou isolated in 1957. The Na+/K+-ATPase was only the first member of a large and still-growing protein family (see Swiss-Prot Prosite motif PS00154).
The Transporter Classification Database provides a representative list of members of the P-ATPase superfamily, which as of early 2016 consisting of 20 families. Members of the P-ATPase superfamily are found in bacteria, archaea and eukaryotes. Clustering on the phylogenetic tree is usually in accordance with specificity for the transported ion(s).
In eukaryotes, they are present in the plasma membranes or endoplasmic reticular membranes. In prokaryotes, they are localized to the cytoplasmic membranes.
Thever & Saier (2009) analyzed the fully sequenced genomes of 26 eukaryotes including animals, plants, fungi and unicellular eukaryotes for P-type ATPases. They reported the organismal distributions, phylogenetic relationships, probable topologies and conserved motifs of nine functionally characterized families and a few uncharacterized families of these enzyme transporters.
Chan et al., (2010) conducted an equivalent but more extensive analysis of the P-type ATPase Superfamily in Prokaryotes and compared them with those from Eukaryotes. While some families are represented in both types of organisms, others are found only in one of the other type. The primary functions of prokaryotic P-type ATPases appear to be protection from environmental stress conditions. Only about half of the P-type ATPase families are functionally characterized.
A phylogenetic analysis of 159 sequences made in 1998 by Axelsen and Palmgren suggested that P-type ATPases can be divided into five subfamilies (types), based strictly on a conserved sequence kernel excluding the highly variable N and C terminal regions. Chan et al. (2010) also analyzed P-type ATPases in all major prokaryotic phyla for which complete genome sequence data were available and compared the results with those for eukaryotic P-type ATPases. The phylogenetic analysis grouped the proteins independent of the organism from which they are isolated and showed that the diversification of the P-type ATPase family occurred prior to the separation of eubacteria, archaea, and eucaryota. This underlines the significance of this protein family for cell survival under stress conditions.
- Type I consists of the transition/heavy metal ATPases. Topological type I (heavy metal) P-type ATPases predominate in prokaryotes (approx. tenfold).
- Type IA ATPases are involved in K+ import (TC# 3.A.3.7). They are atypical P-type ATPases because, unlike other P-type ATPases, they function as part of a heterotetrameric complex (called KdpFABC), where the actual K+ transport is mediated by another subcomponent of the complex.
- Type IB ATPases are involved in transport of the soft Lewis acids: Cu+, Ag+, Cu2+, Zn2+, Cd2+, Pb2+ and Co2+ (TC#s 3.A.3.5 and 3.A.3.6). They are key elements for metal resistance and metal homeostasis in a wide range of organisms.
- Type II ATPases are split into four groups. Topological type II ATPases (specific for Na+,K+, H+ Ca2+, Mg2+ and phospholipids) predominate in eukaryotes (approx. twofold).
- Type IIA transports Ca2+. SERCA1a is a type IIA pump. (TC# 3.A.3.2)
- Type IIB transports Ca2+. (TC# 3.A.3.2)
- Type IIC consists of the closely related Na+/K+ and H+/K+ ATPases from animal cells. (TC# 3.A.3.1)
- Type IID contains a small number of fungal ATPases of unknown function. (Fungal K+ transporters; TC# 3.A.3.9)
- Type III ATPases contains the plasma membrane H+-ATPases from plants and fungi (IIIA) and a small subdivision with Mg2+-ATPases from three bacterial species (IIIB). Fungal H+ transporters (TC# 3.A.3.3) and Mg2+ (TC# 3.A.3.4)
- Type IV ATPases have been shown to be involved in the transport of phospholipids, such as phosphatidylserine, phosphatidylcholine and phosphatidylethanolamine.
- Type V ATPases have unknown specificity. This large group is found only in eukaryotes and is believed to be involved in cation transport in the endoplasmic reticulum.
Horizontal Gene Transfer
Many P-type ATPase families are found exclusively in prokaryotes (e.g. Kdp-type K+ uptake ATPases (type III) and all prokaryotic functionally uncharacterized P-type ATPase (FUPA) families), while others are restricted to eukaryotes (e.g. phospholipid flippases and all 13 eukaryotic FUPA families). Horizontal gene transfer has occurred frequently among bacteria and archaea, which have similar distributions of these enzymes, but rarely between most eukaryotic kingdoms, and even more rarely between eukaryotes and prokaryotes. In some bacterial phyla (e.g. Bacteroidetes, Flavobacteria and Fusobacteria), ATPase gene gain and loss as well as horizontal transfer occurred seldom in contrast to most other bacterial phyla. Some families (i.e., Kdp-type ATPases) underwent far less horizontal gene transfer than other prokaryotic families, possibly due to their multisubunit characteristics. Functional motifs are better conserved across family lines than across organismal lines, and these motifs can be family specific, facilitating functional predictions. In some cases, gene fusion events created P-type ATPases covalently linked to regulatory catalytic enzymes. In one family (FUPA Family 24), a type I ATPase gene (N-terminal) is fused to a type II ATPase gene (C-terminal) with retention of function only for the latter. Genome minimalization led to preferential loss of P-type ATPase genes. Chan et al. (2010) suggested that in prokaryotes and some unicellular eukaryotes, the primary function of P-type ATPases is protection from extreme environmental stress conditions. The classification of P-type ATPases of unknown function into phylogenetic families provides guides for future molecular biological studies.
Many of these protein complexes are multisubunit with a large subunit serving the primary ATPase and ion translocation functions. Many eukaryotic P-type ATPases are monomeric or homodimeric enzymes of the catalytic subunit that hydrolyzes ATP. They contain the aspartyl phosphorylation site and catalyzes ion transport. The Na+/K+-ATPases, the Ca2+-ATPases and the (fungal) H+-ATPases of higher organisms exhibit 10 transmembrane α helical spanners (TMSs), some of them highly tilted. Additional subunits that appear to lack catalytic activity may be present in the ATPase complex.
The X-ray crystal structure at 3.5 Å resolution of the pig renal Na+/K+-ATPase has been determined with two rubidium ions bound in an occluded state in the transmembrane part of the α-subunit. Several of the residues forming the cavity for rubidium/potassium occlusion in the Na+/K+-ATPase are homologous to those binding calcium in the Ca2+-ATPase of the sarco(endo)plasmic reticulum. The carboxy terminus of the α-subunit is contained within a pocket between transmembrane helices and seems to be a novel regulatory element controlling sodium affinity, possibly influenced by the membrane potential.
Structures are available for both the E1 and E2 states of the Ca2+ ATPase showing that Ca2+ binding induces major changes in all three cytoplasmic domains relative to each other. Xu et al. proposed how Ca2+ binding induces conformational changes in TMS 4 and 5 in the membrane domain (M) that in turn induce rotation of the phosphorylation domain (P). The nucleotide binding (N) and β-sheet (β) domains are highly mobile, with N flexibly linked to P, and β flexibly linked to M. Modeling of the fungal H+ ATPase, based on the structures of the Ca2+ pump, suggested a comparable 70º rotation of N relative to P to deliver ATP to the phosphorylation site.
Crystal structures have shown that the conserved TGES loop of the Ca2+-ATPase is isolated in the Ca2E1 state but becomes inserted in the catalytic site in E2 states. Anthonisen et al. (2006) characterized the kinetics of the partial reaction steps of the transport cycle and the binding of the phosphoryl analogs BeF, AlF, MgF, and vanadate in mutants with alterations to conserved TGES loop residues. The data provide functional evidence supporting a role of Glu183 in activating the water molecule involved in the E2P → E2 dephosphorylation and suggest a direct participation of the side chains of the TGES loop in the control and facilitation of the insertion of the loop in the catalytic site. The interactions of the TGES loop furthermore seem to facilitate its disengagement from the catalytic site during the E2 → Ca2E1 transition.
Most of our knowledge about the structure and function of P-type ATPases originates from SERCA1a, a sarco(endo)plasmic reticulum Ca2+-ATPase of fast twitch muscle from adult rabbit. It is generally acknowledged that the structure of SERCA1a is representative for the superfamily of P-type ATPases.
SERCA1a is composed of a cytoplasmic section and a transmembrane section with two Ca2+-binding sites. The cytoplasmic section consists of three cytoplasmic domains, designated the P, N, and A domains, containing over half the mass of the protein. The transmembrane section has ten transmembrane helices (M1-M10), with the two Ca2+-binding sites located near the midpoint of the bilayer. The binding sites are formed by side-chains and backbone carbonyls from M4, M5, M6, and M8. M4 is unwound in this region due to a conserved proline (P308). This unwinding of M4 is recognised as a key structural feature of P-type ATPases.
The P domain contains the canonical aspartic acid phosphorylated during the reaction cycle. It is composed of two parts widely separated in sequence. These two parts assemble into a seven-strand parallel β-sheet with eight short associated a-helices, forming a Rossmann fold.
The N domain is inserted between the two segments of the P domain, and is formed of a seven-strand antiparallel β-sheet between two helix bundles. This domain contains the ATP-binding pocket, pointing out toward the solvent near the P-domain.
The A domain is the smallest of the three domains. It consists of a distorted jellyroll structure and two short helices. It is the actuator domain modulating the occlusion of Ca2+ in the transmembrane binding sites, and it is pivot in transposing the energy from the hydrolysis of ATP in the cytoplasmic domains to the vectorial transport of cations in the transmembrane domain. The A domain dephosphorylates the P domain as part of the reaction cycle using a highly conserved TGES motif located at one end of the jellyroll.
ATP hydrolysis occurs in the cytoplasmic headpiece at the interface between domain N and P. Two Mg-ion sites form part of the active site. ATP hydrolysis is tightly coupled to Ca2+ translocation through the membrane, more than 40 Å away, by the A domain.
It is interesting to note that the folding pattern and the locations of the critical amino acids for phosphorylation in P-type ATPases has the haloacid dehalogenase fold characteristic of the haloacid dehalogenase (HAD) superfamily, as predicted by sequence homology. The HAD superfamily functions on the common theme of an aspartate ester formation by an SN2 reaction mechanism. This SN2 reaction is clearly observed in the solved structure of SERCA with ADP plus AlF4−.
Differences from SERCA1a
Various subfamilies of P-type ATPases also need additional subunits for proper function. Both P-type IA and P- type IV pumps needs extra subunits to function. The functional unit of Na+/K+-ATPase consists of two additional subunits, beta and gamma, involved in trafficking, folding, and regulation of these pumps. SERCA1a and other P-IIA ATPases are also regulated by phospholamban and sarcolipin in vivo. It is presumed that other subfamilies need additional subunits for the proper function in vivo, also.
Some members of the family have additional domains fused to the pump. Heavy metal pumps can have several N- and C-terminal heavy metal-binding domains that have been found to be involved in regulation.
The proton pumps (IIIA) have a C-terminal regulatory domain (called the R domain), which, when unphosphorylated, inhibit pumping.
While most subfamilies have 10 transmembrane helices, there are some notable exceptions. The P-type IA ATPases are predicted to have 7, and the large subfamily of heavy metal pumps (type IB) is predicted to have 8 transmembrane helices. Type V appears to have a total of 12 transmembrane helices.
Proton pumps in the plasma membrane of plants and yeasts maintain the intracellular pH and membrane potential. Kühlbrandt et al. (2002) built an atomic homology model of the proton pump based on the 2.6 angstrom x-ray structure of the related Ca2+ pump from rabbit sarcoplasmic reticulum in an attempt to learn about the molecular mechanisms of proton pumping. The model, when fitted to an 8 angstrom map of the Neurospora proton pump determined by electron microscopy, reveals the likely path of the proton through the membrane and shows that the nucleotide-binding domain rotates by ∼70° to deliver adenosine triphosphate (ATP) to the phosphorylation site. A synthetic peptide corresponding to the carboxyl-terminal regulatory domain stimulates ATPase activity, suggesting a mechanism for proton transport regulation. Ten transmembrane helices and three cytoplasmic domains define the functional unit of ATP-coupled proton transport across the plasma membrane, and the structure is locked in a functional state not previously observed in P-type ATPases. The transmembrane domain reveals a large cavity, which is likely to be filled with water, located near the middle of the membrane plane where it is lined by conserved hydrophilic and charged residues. Proton transport against a high membrane potential is readily explained by this structural arrangement.
P-type ATPases play essential roles in numerous processes, which in humans include nerve impulse propagation, relaxation of muscle fibers, secretion and absorption in the kidney, acidification of the stomach and nutrient absorption in the intestine.
All P-type ATPases use the energy derived from ATP to drive transport. They generally form a high-energy aspartyl-phosphoanhydride intermediate in the reaction cycle, and they interconvert between at least two different conformations, denoted by E1 and E2. The E1-E2 notation stems from the initial studies on this family of enzymes made on the Na+/K+-ATPase, where the sodium form and the potassium form are referred to as E1 and E2, respectively, in the "Post-Albers scheme". The E1-E2 schema has been proven to work, but there exist more than two major conformational states. The E1-E2 notation highlights the selectivity of the enzyme. In E1, the pump has high affinity for the exported substrate and low affinity for the imported substrate. In E2, it has low affinity of the exported substrate and high affinity for the imported substrate. Four major enzyme states form the cornerstones in the reaction cycle. Several additional reaction intermediates occur interposed. These are termed E1~P, E2P, E2-P*, and E1/E2.
In the case of SERCA1a, energy from ATP is used to transport 2 Ca2+-ions from the cytoplasmic side to the lumen of the sarcoplasmatic reticulum, and to countertransport 1-3 protons into the cytoplasm. Starting in the E1/E2 state, the reaction cycle begins as the enzyme releases 1-3 protons from the cation-ligating residues, in exchange for cytoplasmic Ca2+-ions. This leads to assembly of the phosphorylation site between the ATP-bound N domain and the P domain, while the A domain directs the occlusion of the bound Ca2+. In this occluded state, the Ca2+ ions are buried in a proteinaceous environment with no access to either side of the membrane.The Ca2E1~P state becomes formed through a kinase reaction, where the P domain becomes phosphorylated, producing ADP. The cleavage of the β-phosphodiester bond releases the gamma-phosphate from ADP and unleashes the N domain from the P domain.
This then allows the A domain to rotate toward the phosphorylation site, making a firm association with both the P and the N domains. This movement of the A domain exerts a downward push on M3-M4 and a drag on M1-M2, forcing the pump to open at the luminal side and forming the E2P state. During this transition, the transmembrane Ca2+-binding residues are forced apart, destroying the high-affinity binding site. This is in agreement with the general model form substrate translocation, showing that energy in primary transport is not used to bind the substrate but to release it again from the buried counter ions. At the same time the N domain becomes exposed to the cytosol, ready for ATP exchange at the nucleotide-binding site.
As the Ca2+ dissociate to the luminal side, the cation binding sites are neutralised by proton binding, which makes a closure of the transmembrane segments favourable. This closure is coupled to a downward rotation of the A domain and a movement of the P domain, which then leads to the E2-P* occluded state. Meanwhile, the N domain exchanges ADP for ATP.
The P domain is dephosphorylated by the A domain, and the cycle completes when the phosphate is released from the enzyme, stimulated by the newly bound ATP, while a cytoplasmic pathway opens to exchange the protons for two new Ca2+-ions.
Metal binding to transmembrane metal-binding sites (TM-MBS) in Cu+-ATPases is required for enzyme phosphorylation and subsequent transport. However, Cu+ does not access Cu+-ATPases in a free (hydrated) form but is bound to a chaperone protein. The delivery of Cu+ by Archaeoglobus fulgidus Cu+-chaperone, CopZ (see TC# 3.A.3.5.7), to the corresponding Cu+-ATPase, CopA (TC# 3.A.3.5.30), has been studied. CopZ interacted with and delivered the metal to the N-terminal metal binding domain(s) of CopA (MBDs). Cu+-loaded MBDs, acting as metal donors, were unable to activate CopA or a truncated CopA lacking MBDs. Conversely, Cu+-loaded CopZ activated the CopA ATPase and CopA constructs in which MBDs were rendered unable to bind Cu+. Furthermore, under nonturnover conditions, CopZ transferred Cu+ to the TM-MBS of a CopA lacking MBDs altogether. Thus, MBDs may serve a regulatory function without participating directly in metal transport, and the chaperone delivers Cu+ directly to transmembrane transport sites of Cu+-ATPases. Wu et al. (2008) have determined structures of two constructs of the Cu (CopA) pump from Archaeoglobus fulgidus by cryoelectron microscopy of tubular crystals, which revealed the overall architecture and domain organization of the molecule. They localized its N-terminal MBD within the cytoplasmic domains that use ATP hydrolysis to drive the transport cycle and built a pseudoatomic model by fitting existing crystallographic structures into the cryoelectron microscopy maps for CopA. The results also similarly suggested a Cu-dependent regulatory role for the MBD.
In the Archaeoglobus fulgidus CopA (TC# 3.A.3.5.7), invariant residues in helixes 6, 7 and 8 form two transmembrane metal binding sites (TM-MBSs). These bind Cu+ with high affinity in a trigonal planar geometry. The cytoplasmic Cu+ chaperone CopZ transfers the metal directly to the TM-MBSs; however, loading both of the TM-MBSs requires binding of nucleotides to the enzyme. In agreement with the classical transport mechanism of P-type ATPases, occupancy of both transmembrane sites by cytoplasmic Cu+ is a requirement for enzyme phosphorylation and subsequent transport into the periplasmic or extracellular milieu. Transport studies have shown that most Cu+-ATPases drive cytoplasmic Cu+ efflux, albeit with quite different transport rates in tune with their various physiological roles. Archetypical Cu+-efflux pumps responsible for Cu+ tolerance, like the Escherichia coli CopA, have turnover rates ten times higher than those involved in cuproprotein assembly (or alternative functions). This explains the incapability of the latter group to significantly contribute to the metal efflux required for survival in high copper environments. Structural and mechanistic details of copper-transporting P-type ATPase functionhave been described.
General transport reaction
The generalized reaction for P-type ATPases is:
nMe1 (out) + mMe2 (in) + ATP → nMe1 (in) + mMe2 (out) + ADP + Pi.
Human genes encoding P-type ATPases or P-type ATPase-like proteins include:
- Na+/K+ transporting: ATP1A1, ATP1A2, ATP1A3, ATP1A4, ATP1B1, ATP1B2, ATP1B3, ATP1B4
- Ca++ transporting: ATP2A1, ATP2A2, ATP2A3, ATP2B1, ATP2B2, ATP2B3, ATP2B4, ATP2C1
- Cu++ transporting: ATP7A, ATP7B
- Class I, type 8: ATP8A1, ATP8B1, ATP8B2, ATP8B3, ATP8B4
- Class II, type 9: ATP9A, ATP9B
- Class V, type 10: ATP10A, ATP10B, ATP10D
- Class IV, type 11: ATP11A, ATP11B, ATP11C
- H+/K+ transporting, gastric: ATP4A;
- H+/K+ transporting, nongastric: ATP12A
- type 13: ATP13A1, ATP13A2, ATP13A3, ATP13A4, ATP13A5
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